If 'Pair of Tidds' makes you think the name is a euphemism, you're not alone.
Tidds are a simple product designed to solve a simple problem. What do you do with bottle cage bosses when you don't want to put down a bottle cage? Some people leave them empty. Tidds says this is a bad thing, leaving your bosses exposed to the elements, and allowing water, dirt, and the elements to enter your frame. Others use standard bolts to seal the holes. That's fine says Tidds, except you don't need to use that much weight. Each Tidd is just a quarter of a gram, a good tenth of the weight of the typical steel bolt. You can insert these silicon Tidds into your frame's bottle bosses, to cover the holes and save weight.
In practice, they work just fine: you apply some grease to the threads, then push and twist them into place in your frame's bottle bosses. They'll twist into a position that's flush, or close to it (you can improve the flushness by by cutting off the tip of the Tidd). It's a nice solution, as long as you don't mind paying about six bucks per pair (3.99 Pounds Sterling). For about the same price, you can get a pair of aluminum M5 screws, which can both cover the bosses AND serve as bottle cage bolts, should you choose to add a bottle cage later on. The drawback is that even the aluminum screws, at 1 gram each, are heavier than than the 0.25g Tidds. Of course, to most people that won't matter one bit. For weight weenies, 2g per pair versus 0.5g per pair might make a difference. I leave that choice to you.
If you're interested in Tidds, you can get them for yourself at SiliconeTidds.com